10 + 9.9 - 9.8 - 9.7 +...+ 0.4 + 0.3 - 0.2 - 0.1
= (10 + 9.9 - 9.8 - 9.7) +...+ (0.4 + 0.3 - 0.2 - 0.1)
= 0.4×25
= 10